1. Field of the Invention
This invention relates to a disc for recording information signals, such as an optical disc or a magneto-optical disc, referred to hereinafter as an optical disc, and a method for producing the same.
2. Description of the Related Art
There has hitherto been proposed a disc for recording information signals, such as an optical disc or a magneto-optical disc.
With such disc, information signals can be recorded to a higher density, and hence a disc of an extremely small size has been proposed. For example, a small-sized magneto-optical disc 64 mm or less in diameter has been proposed.
The magneto-optical disc, which is small-sized and capable of recording to a higher density, is loaded on a disc rotating and driving device for being rotated at a higher velocity. While the disc is rotated at a higher velocity, a light beam radiated from an optical pickup is directed to a recording track provided on a signal recording layer formed on a major surface of the disc, while an external magnetic field is applied by the magnetic head for recording desired information signals on the disc.
For accurately radiating the light beam on the recording track of the magneto-optical disc rotated at an elevated velocity, it is necessary for the disc to be positively unified with a disc table of the disc rotating and driving device and to be positioned and loaded highly accurately with respect to the disc table.
Meanwhile, for positively unifying the magneto-optical disc to the disc table and loading the disc accurately with respect to the disc table, a disc loading system has been proposed in e.g. U.S. Pat. Nos. 4,787,009 and 4,829,510, according to which, with the magneto-optical disc set on the disc table, a chucking magnetic plate of e.g. a magnetic metal material, placed at the center of the magneto-optical disc, is magnetically attracted by a permanent magnet provided on the disc table, for loading the magneto-optical disc on the disc table.
With the magneto-optical disc 1, employed in the disc loading system taking advantage of the magnetic attraction by the permanent magnet, a metal plate 3 as a magnetic plate is placed for closing a center opening 2 formed in the disc 1, as shown in FIG. 1. A disc rotating and driving device 4 for rotationally driving the magneto-optical disc 1 includes a spindle motor 5 having a rotary shaft 6 integrally carrying a disc table 7 and a permanent magnet 9 integrally mounted on the distal end of the rotary shaft 6 by means of a magnet holder 8. A centering member 11 is mounted at the center of the disc table 7 by being housed within a housing recess 10 formed at the center of the disc table, and is biased by a coil spring 12 for being moved axially of the rotary shaft 6.
Referring to FIG. 1, the magneto-optical disc 1 is set on the disc table 7, with the centering member 11 engaged in the center opening 2 with the rim of the centering hole 2 being supported by the disc table 7. The magneto-optical disc 1 is loaded as one with the disc table 7 by the metal plate 3 being magnetically attracted by the magnet 9. When set and loaded on the disc table 7, the magneto-optical disc 1 has its center of rotation aligned with the axis of the rotary shaft 6 by the centering member 11 which is introduced into engagement with the centering opening 2 while being moved axially of the rotary shaft 6.
By thus attracting the metal plate 3 on the magneto-optical disc 1 by the magnet 9 of the disc table 7, and centering the disc by the centering member 11, when setting the magneto-optical disc 1 on the disc table 7, the disc 1 may be loaded on the disc table 7 as one with and with highly accurate positioning relative to the disc table 7. On the other hand, the chucking mechanism of positioning and integrally loading the magneto-optical disc 1 on the disc table may be simplified in structure.
Meanwhile, the magneto-optical disc 1, which is of a reduced diameter and permits of high density recording of information signals, is housed in the disc cartridge 13, and loaded in this state on the disc rotating and driving device 4.
Meanwhile, the previously proposed metal plate mounted on the magneto-optical disc is bonded to a base plate or substrate of the magneto-optical disc, formed of a synthetic resin, such as a transparent polycarbonate resin, with the aid of a UV curable adhesive or a double-sided adhesive tape, in a manner free from positional deviation on the disc substrate.
However, since the thermal expansion coefficient of the metal plate differs markedly from that of the disc base plate formed of synthetic resin, if the metal plate is joined to the disc base plate with the aid of the above-mentioned adhesive, the metal plate may be peeled off from the disc base plate on repeated expansion and contraction of the metal plate and the disc base plate due to changes in ambient temperatures. On the other hand, if the metal plate and the disc base plate are secured strongly to each other in a manner free from peeling on repeated expansion and contraction, distortion may be produced in the disc base plate with changes in the ambient temperature, so that double refraction is produced within the disc base plate and hence information signals cannot be recorded or reproduced with good recording/reproducing properties on or from the signal recording layer formed on the surface of the disc base plate.
Above all, if the metal plate is bonded to the disc base plate with the aid of a UV curable adhesive, the risk is extremely high that double refraction be produced within the disc base plate during curing of the adhesive. If the UV curable adhesive is employed, it becomes difficult to improve the productivity of the magneto-optical disc because of the curing time involved in curing the adhesive.
On the other hand, since the double-sided adhesive tape is lacking in bonding strength, sufficient reliability in the bonding between the metal plate and the disc base plate cannot be assured with the use of such adhesive tape.